Xerographic development apparatus and process

ABSTRACT

Electroscopic toner material is applied to electrostatic charge patterns with at least one metallized fur brush having individual flexible filaments coated with a thin layer of an electrically conductive metal. The apparatus contains two metallized fur brushes, one having a low electrical conductivity and one having a high conductivity.

United States Patent Miller [54] XEROGRAPHIC DEVELOPMENT APPARATUS ANDPROCESS [72] Inventor: Howard A. Miller, Rochester, NY.

[73] Assignee: Eastman Kodak Company, Rochester,

[22] Filed: Feb. 6, 1970 [21] Appl No.: 9,225

[52] US. Cl. ..ll7/17.5, 118/637, 117/111 C [51] Int. Cl. t ..G03g13/08, 603g 15/08 [58] Field ofSearch ..117/17.5, 111 C; 118/637 [56]References Cited UNITED STATES PATENTS 3,542,579 1 1/1970 Gundlach..118/637 X 2,927,554 3/1960 Oldenboom. ..118/637 3,375,806 4/1968 Nost..118/637 [15] 3,664,857 [451 May 23, 19 72 3,251,706 5/1966 Walkup..1l7/17.5 3,492,151 1/1970 Cescon ..117/160X 3,097,962 7/1963 Whitacre..117/160 X 3,420,151 l/l969 Levine et a]. ..355/4 2,902,974 9/1959Greaves ..l l7/l7.5 X 3,357,402 12/1967 Bhaget ..1 l7/l7.5 X

Primary ExaminerWilliam D. Martin Assistant ExaminerM. SofocleousAttorney-W. H. .l. Kline, J. R. Frederick and T. Hiatt [57] ABSTRACTElectroscopic toner material is applied to electrostatic charge patternswith at least one metallized fur brush having individual flexiblefilaments coated with a thin layer of an electrically conductive metal.The apparatus contains two metallized fur brushes, one having a lowelectrical conductivity and one having a high conductivity.

10 Claims, 2 Drawing Figures PATENTEDmza I972 3, 664, 857

FIG.

HOWARD A. MILLER INVENTOR.

'TTS ATTORNEY XEROGRAPHIC DEVELOPMENT APPARATUS AND PROCESS Thisinvention relates to electrography, and more particularly, to thedevelopment of electrostatic latent images.

Electrophotographic imaging processes and techniques have beenextensively described in both the patent and other literature, forexample, U. S. Pat. Nos. 2,221,776; 2,277,013; 2,297,691; 2,357,809;2,551,582; 2,825,814; 2,833,648; 3,220,324; 3,220,831; 3,220,833 andmany others. Generally, these processes have in common the steps ofemploying a normally insulating photoconductive element which isprepared to respond to image-wise exposure with electromagneticradiation by forming a latent electrostatic charge image. Theelectrostatic latent image is then rendered visible by a developmentstep in which the charged surface of the photoconductive element isbrought into contact with a suitable developer mix.

One method of accomplishing development has been to cascade across theimage-bearing surface a two-component developer mix as described in U.S. Pat. No. 2,618,551. This cascade development system is useful forordinary line copies; however, it has limited application where solidarea development is required. Consequently, workers in the art devisedsystems for magnetic brush development which provide improved solid areadevelopment.

In a magnetic brush system, a developer mix typically comprised of ironcarrier particles and colored resin toner particles is applied to anapparatus of the type described in U. S. Pat. No. 3,003,462. In such anapparatus, the iron particles are held by a magnet in a bristle-likeformation resembling a brush with the toner particles adhering byelectrostatic attraction. These bristles of iron particles areelectrically conductive which provides the effect of a developmentelectrode having a very close spacing to the element being developed.

The development electrode effect obtained in magnetic brush developmentis advantageous where it is desired to copy images having large solidareas. However, there are several disadvantages associated with magneticbrush development. The bristles formed in a magnetic brush arerelatively stiff and cause gradual wear on reusable charge-bearingelements. Also the stiff bristles often form streaks in the developedimages. A further disadvantage arises from the particulate nature of themagnetic carrier. Loose carrier particles are often thrown off themagnetic brush and result in unwanted background density in theresultant images.

An alternative method of dry development has been proposed whichinvolves the use of a fur brush. The fur brush method of development hasan important advantage over the more common cascade and magnetic brushmodes of development in that the carrier is an integral unit which canbe fully controlled. However, for the most part, the use of the furbrush carrier is limited to so-called fringing development.

Furthermore, although convenient and relatively simple, fur brushdevelopment has been seriously restricted because of difficulty ingrounding and electrically biasing the brush. These difficulties arisebecause of local differences in the electrical characteristics acrossthe brush surface. This problem is magnified by the humidity dependentelectrical characteristics of previous brushes.

When used at low relative humidity, previous brushes accumulate staticcharge which tend to bind the toner particles more tightly to the brushwhich results in less toner being applied to the charge pattern.Accordingly, the image density is considerably reduced. On the otherhand, at high relative humidity the reverse is true in that excessivebackground density is produced which in some cases completelyobliterates the developed image. Most prior art references to fur brushdevelopment contain specific warnings as to the problem associated withexcessive conductivity.

Accordingly, there is a need in the art for a fur brush system ofdeveloping which eliminates the problems of humidity dependentconductivity, unwanted charge buildup, and grounding and biasingdifficulties. In addition, there is a need for a system which permitsthe operator to select either a continuous tone or a fringingdevelopment mode from a single apparatus by the use of a simplemechanical switching.

It is, therefore, an object of this invention to provide an improved drysystem for developing electrostatic charge patterns, which system iscapable of producing solid area and/or fringing development.

It is another object of this invention to provide dry developmentapparatus utilizing fur brush toner applicators.

It is still another object of this invention to provide means for drydevelopment of electrostatic charge patterns in a manner which providesreproducible characteristics that are independent of relative humidity.

These and other objects of the invention are achieved by the use of anapparatus for the dry development of electrostatic charge patterns whichapparatus contains two toner applicators in the form of humidityinsensitive, fur brush units. One of these brush units has a highelectrical conductivity and the other has a low electrical conductivity.The fur brushes of this invention are in the form of a metallized furmaterial comprising soft flexible metallized bristles or filamentssupported on a conducting support such as a core or web. This supportprovides an electrical connection between the individual electricallyconducting metallized filaments. Such a brush can take the conventionalform of a plurality of filaments held firmly by and projecting radiallyfrom a rigid spirally twisted wire support forming the longitudinalaxis. The brush can also comprise a pile or coat of filaments eachfirmly attached at one or both ends to an underlying backing web. Suchmetallized fur brushes, including synthetic fur or pile fabrics andsimilarly constituted natural furs can be used in a fur brushdevelopment in the form of a covering over a rotating cylinder, as anendless belt running over a plurality of rollers and the like.

The invention will be described in more detail with reference to thefigures in which:

FIG. 1 is a schematic cross section of a brush of this invention, and vFIG. 2 is a schematic cross section of an embodiment of the apparatus ofthis invention.

Referring now to FIG. 1, it is seen that the metallized fur brush 11 istypically comprised of a pile formed of a multitude of individualmetallized fibers or filaments 12 carried on su port 13. This support 13can be a woven substrate as in the case of a synthetic fur material orit can be a natural material as in the case ofanimal furs. Support 13 isalso typically metal lized such that it is conducting. Additionally,support 13 can be in physical and electrical contact with anelectrically conducting backing member 14 in order to provide suitablemechanical strength, etc. The actual form of this metallized fur brushcan be varied to suit individual need. For example, the brush can be inthe form of a flat platen. a cylindrical roller, a continuous belt, etc.In addition, the brush can also take the form of a test tube brush inwhich case support 13 would be in the form of twisted wire or otherstiff material.

The brushes of this invention can be prepared from a variety of naturaland synthetic fur materials. Suitable natural or animal furs wouldinclude rabbit, fox, beaver, skunk, and raccoon furs,and the like.Useful materials for preparing the synthetic fur brushes in accordancewith this invention would include poly-a-olefins such as polyethylene,polypropylene, and others prepared from olefins containing from two to10 carbon atoms; polyesters such as poly(ethylene terephthalate),polyesters of 1,4-cyclohexanedimethanol and terephthalic acid asdescribed in U. S. Pat. No. 2,901,466; polyamides such as variousnylons, for example, poly(hexamethylene adipamide), polycaprolactam,poly(hexamethylene sebacamide), etc; fiber-forming resins comprising'copolymers of vinylidene chloride and acrylonitrile as dislimited onlyby the amount ofa flexibility desired. In general, the filaments have auseful range of thickness from about 0.002 to about 1 mm. with apreferred thickness in the range of about 0.0025 to about 0.75 mm. Ofcourse, thicker filaments can be used advantageously in certainapplications and particularly when the outermost end of the filament isflattened or split to provide soft, flexible contact with aphotoconductive element. Smaller diameter filaments can also be usedwhere desired with the smallest thickness theoretically being limitedonly b the capabilities of filament forming techniques. In addition, thelength of the individual filaments, as measured from the support to theoutermost portion of the filament, can vary over a wide range. Usefulresults are obtained with filaments ranging in in length from about /2to about 4 cm., with preferred materials having a length of about 1 toabout 3% cm. The lower limit offilament length is limited by theflexibility desired.

The natural or synthetic fur materials in accordance with this inventionare given a coating of an electrically conducting metal. The metal isdeposited as a thin, continuously conducting, adherent layer on eachindividual filament forming the brush. This metal film renders theindividual filaments electrically conductive independentlv of therelative humidity. Such was not the case with many previous fur brushes,as witnessed by the apparatus necessary for maintaining a given relativehumidity in the fur brush development assembly described in U. S. Pat.No. 2,902,974. In conventional fur brush development (fringingdevelopment) some leakage path must be provided to prevent accumulationof excessive charge. This is done by treating bristles or fur withantistatic compound which requires some moisture in the ambient air toinsure conductivity; hence, relative humidity is important in using suchbrushes. However, one should not equate such humidity dependentconductivity with the humidity independent conductivity of the brushesof this invention.

Others have attempted to prepare brushes made of solid metal fibers;however, such attempts have not been entirely successful because of therelative stiffness of the resultant brush. I have found that a startingfur-like material can be chosen for its mechanical properties, i.e.,softness and flexibility, and be metallized in accordance with thisinvention without substantial deterioration of the desired mechanicalproperties. In addition, suitable starting furs can be chosen withoutregard for their triboelectric properties in that the metallizationtechnique described herein imparts triboelectric properties independentof the inherent properties of the starting material.

The fur materials useful in this invention are coated with a tightlyadherent layer of an electrically conductive metal. Useful materials forforming these electrically conductive coatings include those metals ingroups VIa, VIII, Ib and IIb of the Periodic Table (Cotton andWilkinson, Advanced Inorganic Chemistry, l962, page 30) and aluminum.Particularly useful metals are aluminum, cadmium, chromium, cobalt,copper, gold, iron, nickel, silver, zinc, and the platinum groupelements which include ruthenium, rhodium, palladium, osmium, iridium,and platinum as well as mixtures or alloys of any of these.

The metallization of any of the useful fur materials can be accomplishedby various means such as vacuum evaporation, electroplating, electrolessdeposition, etc. In addition, combinations of the various procedures canbe used. In the case of synthetic furs, the finished fur material can bemetallized or the synthetic material could be metallized duringmanufacture of the filaments, e.g., immediatelv after extrusion, andthen formed into a pile fabric, etc. The thickness of the metal coatingcan vary considerably provided that the coating does not become so thickas to make the filaments too stiff. In addition, sufficient metal shouldbe deposited so as to provide a continuously conducting layer.Generally, the thickness of the metal will range from about 0.01 toabout 1 micron, with a preferred range of from about 0.02 to about 0.2micron. Useful metallized fur materials are described further incopending U. S. Application Ser. No. 9457, filed Feb. 6, 1970, entitledMETALLIZED FUR MATERIALS, in the names of Miller and Ville.

A particularly useful method for metallizing the fur materials is theprocess of electroless deposition. In accordance with this process, thefur is sensitized, for example, by successive treatments in dilutestannous chloride and dilute palladium chloride to form a thin coatingof palladium. The thin coating acts as a catalyst for the subsequentdeposition of, for example, nickel from an alkaline electroless nickelplating solution typically comprising nickel chloride, sodium citrate,ammonium chloride, sodium hvpophosphite and ammonia. Such an electrolessdeposition technique can also be used to deposit copper, cobalt,palladium, etc.

In addition, a suitably adherent, continuously conducting layer of metalcan readily be applied by vacuum evaporation. A convenient way ofaccomplishing this technique is to comb the filaments or fibers in onedirection and subject the fur to vacuum deposition of, for example,aluminum. The fur is then combed in another direction and subjectedagain to vacuum deposition of aluminum. This process can be repeated asmany times as necessary to provide sufficient conductivity.

In accordance with this invention, the apparatus contains two suchmetallized fur brushes which can be used as a carrier for dryparticulate toner material. FIG. 2 shows an arrangement suitable forapplying toner 22to metallized fur brushes 23 and 24 in the form ofcylindrical rollers. Toner 22 is contained in a reservoir at the bottomof housing 21. The toner is applied to the rotating brushes by means ofa toner applicator means 25 which is comprised of a stiff bristle brushwhich rotates at about 1 /2 to 3 times the peripheral speed of the furbrushes thereby applying a uniform coating of toner 22 to the pile ofthe brushes. Fur brushes 23 and 24 are comprised of a plurality offilaments l2 and 12' carried on a suitable support. The supports areunderlaid by electrically conductive backings l4 and 14. Theelectrically conductive backings are supporting cylinders 14 and 14which are electrically insulated from one another and from ground.Cylinders l4 and 14 can be provided with conducting leads 26 and 27which can be selectively connected to ground or to suitable circuitry(not shown) to provide or control electrical biasing. The fur brushes 23and 24 are rotatably mounted and are driven by drive means not shown.

A charge-bearing element 20 is conveyed by means 28, 29 and 30 shown forsimplicity as being movable roller guides. In the particular embodimentshown guide means 29 is rotatably mounted in a stationary positionwhereas guide means 28 and 30 are mounted so as to be selectivelymovable. With guide means 28 in a lower position and guide means 30 inthe upper position, element 20 will be brought in contact with brush 24which has a higher electrical resistivity than brush 23. This results ina fringe developed image. When guide means 28 is in the upper positionand guide means 30 in the lower position, element 20 will be brought incontact with brush 23 which has a lower electrical resistivity. Thisresults in a solid area developed image. In the third mode of operationguide means 28 and 30 are both maintained in the lower position whichresults in element 20 being placed sequentially in contact with brush 24and brush 23. In this mode, the electrostatic charge pattern issubjected to both fringing and solid area development.

The metallized fur brushes can be electrically connected to a conductivebacking on element 20. By electrically connecting, for example, the highconductivity fur brush 23 to element 20, the brush acts as a closelyspaced development electrode. This development electrode results in anintensification of the electric field of the electrostatic chargepattern on element 20. This alteration of the electric field during thedevelopment step results in the formation ofa high quality,continuous-tone or solid-area developed image. The development electrodecan also be used to compensate for incorrect exposure or to alter toningcharacteristics, etc, by applying positive or negative potential to theelectrode during development.

The metallized fur brush apparatus of the present invention has severaladvantages over any previous brush arrangement. In accordance with thisinvention, a fur material can be selected for its intrinsic softness orflexibility without regard for its conductivity or triboelectriccharacteristics. A flexible fur material can then be made suitablyconductive by metallizing techniques as described below. Equallyimportant is the fact that the conductivity of the resultant flexiblemetallized fur is entirely independent of the relative humidity.Additionally, the conductivity is independent of the amount of toner onthe brush. This is not so in the case of conductive magnetic brusharrangement. In such an arrangement, the toner material tends to coatthe individual magnetic particles.

This coating results in poor electrical contact between adjacentconductive carrier particles. Consequently, the conductivity of such amagnetic brush arrangement tends to decrease with increased use andincreased amounts of toner present. However, the unitary structure ofeach conductive filament of the present metallized fur brushes avoidthis problem found in magnetic brush apparatus.

The present apparatus has another distinct advantage over other furbrush and magnetic brush development apparatus in that both solid areaand fringing development can be readily obtained by a simple mechanicalswitching mechanism. This is accomplished without the need for twoseparate toner hoppers and/or two distinct toner compositions. On thecontrary, the instant invention allows for the production of bothsolid-area and/or fringe developed images from the same tonercomposition.

For continuous-tone or solid-area development, one of the metallized furbrushes must have a sufficiently high, humidityindependent level ofconductivity to permit the brush to function effectively as adevelopment electrode. The resistivity of such a brush is preferably inthe range of about 0.01 ohm/square to about 10,000 ohms/square. Theelectrical resistivity is typically measured on the pile surface of thebrush using two 1 inch by 1/16 inch electrodes which are placed parallelto one another spaced apart about 1 inch while held against the pilesurface with approximately 5 pounds hand pressure. Resistivity valueseven higher than 104 ohms/square can be used to give good solid areadevelopment under appropriate development conditions. However,resistivity values of 100 ohms/square or lower require less criticaldeveloping conditions and are preferred in that this facilitatesdevelopment at higher transit speed and prolongs the useful life of thebrush between cleaning periods. Electrical resistance between thedeveloping surface of the brush and the backside of the pile for supportshould be of about the same order as that referred to above. Thisresistance value can be checked by applying electrodes to the front andback of the fur material under about 5 pounds pressure such that thedistance separating the electrodes is approximately equal to thegreatest length of the electrode. If the front to back conductivity isnot sufficiently high, electrical contact can be made directly with thedeveloping or filamentary surface or pile of the brush using anelectrode in the form of a comb, roller, platen and the like.

For use in fringing, edge, or outline development, a metallized furbrush is required which has a low level of conductivity which is uniformand humidity independent. A low conductivity is required tosubstantially reduce or eliminate the backing or development electrodeeffect. At the same time, however, the conductivity must be high enoughto avoid excessive buildup of electrostatic charges which produce aselfbiasing effect. The preferred range of resistivity of thisbrush isfrom about and 10 ohms/square. An optimum value of resistivity canreadily be determined experimentally to suit various parameters of thesystem, such as the circumference of the brush, the length of a furbelt, the relative transit speed against the charge-bearing surface, thebias voltage, resistance to ground, etc. The range of 10" to 10 ohmspermits a selection of the degree of fringing development. Thus,semi-outline development is obtained at 10 to 10 ohms/square and isaccompanied by a greater exposure latitude, improvement in fine-linereproduction and highlight detail and a cleaner background than solidarea development will give. In addition, good, high density fill-in ofsolid areas of up to three-sixteenths inch width can be obtained. Atbrush resistivity of 10 to 10 ohms/square, the development becomesprogressively more fringing with an accompanying increase in exposurelatitude.

Typically, the separate brush units can be cylindrical comprising, forexample, long fibers extended radially from a small diameter axial core.The fur brush unit can also be in the form of a fur material fittedaround a cylinder arranged to rotate on its longitudinal axis, ordisposed as an endless flexible belt moving in a designated path aroundtwo or more rollers or arranged in a flat or curved plane or other formsuited to make effective developing contact with the electrostatic imageto be developed. During use, the pile of the fur brush is saturated withtoner powder and run in light contact with the surface of the elementbearing the electrostatic charge pattern.

For good solid area development, the charge-bearing element passes overthe brush at a rate of about 5 to 40 inches per second while the brushis moved or rotated at a peripheral speed of about 5 to 20 inches persecond. Similar operating conditions are suitable for outlinedevelopment using the low conductivity brush.

As is typical in electrography, charge-bearing element 20 can be anelectrophotographic element comprised of a conductive support carrying alayer of a photoconductive electrically insulating composition. Anelectrostatic charge pattern is produced on such an element b uniformlycharging the photoconductive layer in the dark and exposing the layer toa pattern of activating radiation which causes selective dissipation ofthe charge in the light-exposed areas. In addition, the charge-bearingelement 20 can be comprised simply of a sheet of an electricallyinsulating material such as poly( ethylene terephthalate) in which casean electrostatic charge pattern can be applied thereto by, for example,charge transfer from an electrophotographic element of the type referredto above. The electrostatic charge pattern formed on element 20 isultimately developed bv contacting element 20 with at least one of thetoner-carrying brushes 23 or 24. The element 20 bearing the chargepattern is shown for convenience as a continuous web. However, theelement can be in the form of a roller or a flat plate.

The toner material used with the present metallized fur brush carriercan be selected from a wide variety of colored or colorless powders.Typical toners are comprised of a colorant such as a dye or pigment in athermoplastic resin binder. Suitable toners can be prepared b any of thewell known means such as melt blending or spray drying to producecolored particles having an average diameter from about 15p. to about25p" These toner particles are given a positive or negative charge byfrictional electrification. The particular polarity charge to be givento the toner will, of course, be dependent upon the type of image to bedeveloped. When toner is applied to the metallized fur brush, theparticles cling to the fibers of the bmsh by triboelectric attraction.The toner is then held by the brush until carried to the point ofcontact with the imagebearing element. At this point, electrostaticattraction overcomes the triboelectric attraction and the tonerparticles become deposited on the image-bearing element.

The fur brushes used in one embodiment of the invention are comprised ofmetal rollers covered with a synthetic fur having a dense pile of Type505 Verel modified acrylic fibers having a free length of aboutseven-tenths inch. These brushes are metallized by the techniquedescribed in Example 3 of copending Miller and Ville application Ser.No. 9457, filled Feb. 6, 1970, and entitled METALLIZED FUR MATERI- ALS.The brushes are first catalyzed for electroless nickel deposition byfirst degreasing in an aqueous solution of potassium laurate, rinsing,immersing in a 0.25 percent palladous chloride solution for 2 minutes,followed by immersion in a 0.4 percent hydrazine solution and a waterrinse. The fur material is then placed in a nickel plating bathcontaining the following ingredients:

Nickel chloride 45 gm/l. (NiCI,-6H O) Sodium Citrate 100 gm/l. (Na CH,0,-%H O) Ammonium Chloride 5O gm/l. Sodium Hypophosphitc l0 gm/l. (NuH,PO,-H O) 28% Ammonia Sol. 90 gm/l.

The plating bath is maintained about 120 and 200 F and a pH of about to11. The low conductivity or high resistivity brush is removed from thebath before a thick metal coating is deposited. This brush has anelectrical resistivity of about 6 X 10 ohms/square. The highlyconductive brush is left in the bath for a time sufiicient to deposit athicker nickel coating. This latter brush has an electrical resistivityof about ohms/square.

The reservoir in housing 21 is filled with a toner 22 comprised ofelectroscopic particles having an average diameter of about 8 microns.The toner particles are composed of carbon black, nigrosine, and apolystyrene resin. This toner charges positively on the nickeled furbrushes. Next, a charge-bearing element 20, comprised ofa conductingsupport having coating thereon a photoconductive layer containing apolymeric binder and an organic photoconductor is subjected to anegative corona source. The resultant negatively charged element is thengiven an imagewise exposure to actinic radiation. This results in theformation ofa negative electrostatic charge pattern. Charge-bearingelement 20 is then moved into contact with grounded brush 24 having ahigh resistivity. The brush is rotated at a peripheral speed of about 10inches/second and element 20 is moved at a rate of passage over thebrush of about 20 inches per second. Toner is transferred from brush 24to element 20 in relation to the charge pattern. A good fringe developedimage results.

The above procedure is repeated only with guide means 28 in the upperposition and guide means 30 in the lower position. With the guide meansso located, element 20 is brought into contact with highly conductivebrush 23 which is biased to ISO volts with respect to the conductivebacking of element 20. A direct positive image results which has a highmaximum density with very little unwanted background density.

FIG. 2 shows an embodiment in which movable guide means 28 and 30function to bring element 20 into contact with brushes 23 and 24. Ofcourse, element 20 can move in one plane with brushes 23 and 24 beingmoved up into contact with the element. It is preferred, however, thatbrushes be substantially contained within housing 21 for purposes ofcleanliness.

The present invention can also utilize a single brush in the form of aroller or continuous belt which has a high conductivity portion and alow conductivity portion. In accordance with this embodiment, themovement of element 20 would be synchronized or controlled such thatelement 20 is moved into contact with the desired portion or portions ofthe brush.

The invention has been described in detail with particular reference tocertain preferred embodiments thereof, but it will be understood thatvariations and modifications can be effected within the spirit and scopeof the invention.

I claim:

1. Apparatus for applying electroscopic toner material to an elementbearing an electrostatic charge pattern, said apparatus comprising firstand second fur brushes capable of containing said element, each havingindividual flexible filaments, coated with an adherent, thin layer of anelectrically conductive metal, said first brush having a low electricalconat a temperature between ductivity so as to produce fringingdevelopment and said second brush having a high electrical conductivityso as to produce solid area development, means for applying toner tosaid brushes, means for moving said element into proximity of saidbrushes and means for selectively contacting at least one of saidbrushes with said element while maintaining relative movement betweensaid element and said brush, whereby said toner material is transferredonto areas of said elements in relation to said electrostatic chargepattern.

2. Apparatus as described in claim 1 wherein said first brush has anelectrical resistivity of greater than said about 10* ohms/square andwherein said second brush has an electrical resistivity of less thanabout 10* ohms/square.

3. Apparatus as described in claim 1 wherein said first and secondbrushes are comprised of synthetic or natural furs hav ing coatedthereon a metal selected from the group consisting ofa Group VIa metal,a group VIII metal, a Group Ib metal, a Group IIb metal, aluminum andmixtures thereof.

4. Apparatus as described in claim 1 wherein at least one of saidbrushes is in the form of a cylinder.

5. Apparatus as described in claim 1 wherein at least one of saidbrushes is in the form of a continuous belt.

6. Apparatus as described in claim 1 wherein said means for alternatelycontacting one of said brushes comprises selectively movable guide meansfor guiding said element into contact with at least one of said brushes.

7. Apparatus for applying electroscopic toner material to an elementbearing an electrostatic charge pattern, said apparatus comprising firstand second movable fur brushes capable of contacting said element, meansfor applying toner to said brushes, means for selectively conveying saidelement into contact with at least one of said brushes whereby saidtoner is attracted from said brush to the element in accordance with thecharge pattern, said brushes comprising synthetic or natural furs havingcoated on the individual filaments thereof a thin layer of anelectrically conductive metal imparting to said brushes a humidityindependent electrical conductivity, said first brush having anelectrical resistivity of greater than about 10" ohms/square and saidsecond brush having an electrical resistivity of less than about 10ohms/square.

8. Apparatus as described in claim 7 including means for sequentiallyguiding said element into contact with both of said brushes wherebytoner is attracted to said element from both brushes.

9. A process for developing electrostatic charge patterns comprising thesteps of:

a. forming an electrostatic charge pattern on an element,

b. applying electroscopic toner material to first and second movable furbrushes each comprised ofa support bearing individual flexible filamentswhich are coated with a thin layer of an electrically conductive metal,one of said brushes having an electrical resistivity of greater than 10ohms per square and the other having an electrical resistivity of lessthan about 104 ohms per square, and

c. sequentially contacting said charge pattern with said first andsecond brushes whereby said toner is deposited in accordance with saidcharge pattern to give both fringing and solid area development.

10. A process as described in claim 9 wherein the fur brushes arecomprised of synthetic or natural furs having coated on the individualfilaments thereof a metal selected from the group consisting ofa GroupVIa metal, a Group VIII metal, a Group Ib metal, a Group IIb metal,aluminum and combinations thereof.

2. Apparatus as described in claim 1 wherein said first brush has anelectrical resistivity of greater than said about 108 ohms/square andwherein said second brush has an electrical resistivity of less thanabout 104 ohms/square.
 3. Apparatus as described in claim 1 wherein saidfirst and second brushes are comprised of synthetic or natural furshaving coated thereon a metal selected from the group consisting of aGroup VIa metal, a group VIII metal, a Group Ib metal, a Group IIbmetal, aluminum and mixtures thereof.
 4. Apparatus as described in claim1 wherein at least one of said brushes is in the form of a cylinder. 5.Apparatus as described in claim 1 wherein at least one of said brushesis in the form of a continuous belt.
 6. Apparatus as described in claim1 wherein said means for alternately contacting one of said brushescomprises selectively movable guide means for guiding said element intocontact with at least one of said brushes.
 7. Apparatus for applyingelectroscopic toner material to an element bearing an electrostaticcharge pattern, said apparatus comprising first and second movable furbrushes capable of contacting said element, means for applying toner tosaid brushes, means for selectively conveying said element into contactwith at least one of said brushes whereby said toner is attracted fromsaid brush to the element in accordance with the charge pattern, saidbrushes comprising synthetic or natural furs having coated on theindividual filaments thereof a thin layer of an electrically conductivemetal imparting to said brushes a humidity independent electricalconductivity, said first brush having an electrical resistivity ofgreater than about 108 ohms/square and said second brush having anelectrical resistivity of less than about 104 ohms/square.
 8. Apparatusas described in claim 7 including means for sequentially guiding saidelement into contact with both of said brushes whereby toner isattracted to said element from both brushes.
 9. A process for developingelectrostatic charge patterns comprising the steps of: a. forming anelectrostatic charge pattern on an element, b. applying electroscopictoner material to first and second movable fur brushes each comprised ofa support bearing individual flexible filaments which are coated with athin layer of an electrically conductive metal, one of said brusheshaving an electrical resistivity of greater than 108 ohms per square andthe other having an electrical resistivity of less than about 104 ohmsper square, and c. sequentially contacting said charge pattern with saidfirst and second brushes whereby said toner is deposited in accordancewith said charge pattern to give both fringing and solid areadevelopment.
 10. A process as described in claim 9 wherein the furbrushes are comprised of synthetic or natural furs having coated on theindividual filaments thereof a metal selected from the group consistingof a Group VIa metal, a Group VIII metal, a Group Ib metal, a Group IIbmetal, aluminum and combinations thereof.